Waveguide window for transmission of electromagnetic waves

ABSTRACT

A window for sealing a waveguide gas pressurized to transmit microwave signals comprising: a dielectric plate permeable to the microwave signals, and a mechanical structure hermetically sealing the dielectric plate in the interior of the waveguide and including four openings dimensioned to provide the window with a substantially flat voltage standing wave ratio versus frequency characteristic over a predetermined frequency range, two relatively movable members to permit the dielectric plate to move to compensate for unequal gas pressures on opposite surfaces thereof, and a space to allow movement of the two members to compensate for thermal expansion thereof due to microwave signal loss in the dielectric plate.

United States Patent Ueda et al.

[54] WAVEGUIDE WINDOW FOR TRANSMISSION OF ELECTROMAGNETIC WAVES [72]Inventors: Isao Ueda; Takao Kageyama; Isao Inomata, all of Tokyo, Japan[73] Assignee: Nippon Electric Company, Limited,

Tokyo, Japan [22] Filed: Aug. 21, 1970 [21] App]. No.: 66,040

2,881,401 4/1959 Bondley ..333/98 P 2,900,568 8/1959 Brewster.........333/98 P X 2,903,614 9/1959 Preist et a] ..333/98 P 2,964,71912/1960 Hatch ..333/98 P 3,101,460 8/1963 Walker et al. .....333/98 P3,163,835 12/1964 Scott ..333/98 P 3,387,237 6/1968 Cook ..333/98 1 July4, 1972 FOREIGN PATENTS OR APPLICATIONS 733,655 7/1955 Great Britain..333/98 P OTHER PUBLICATIONS Vogl, et al., Vacuum Tight Windows withWide Band Transmission Characteristics," Rev. of Scientific Instruments,Vol. 36 010- 10- 65,pp. 1439- 1440 Chen RCA, Broadbanding of ResonantType Microwave Output Windows, RCA Review 6- 1954, pp. 204- 229 Chen',T. S. Output Windows for Tunable Magnetrons," Electronics 5 1954, p.170- 173 Primary Examiner-Herman Karl Saalbach Assistant Examiner-Wm.I-I. Punter AuomeyMam & Jangarathis [57] ABSTRACT A window for sealing awaveguide gas pressurized to transmit microwave signals comprising: adielectric plate permeable to the microwave signals, and a mechanicalstructure hermetically sealing the dielectric plate in the interior ofthe waveguide and including four openings dimensioned to provide thewindow with a substantially flat voltage standing wave ratio versusfrequency characteristic over a predetermined frequency range, tworelatively movable members to permit the dielectric plate to move tocompensate for unequal gas pressures on opposite surfaces thereof, and aspace to allow movement of the two members to compensate for thermalexpansion thereof due to microwave signal loss in the dielectric plate.

5 Claims, 6 Drawing Figures Patented July 4, 1972 3,675,165

2 Sheets-Sheet 1 Fig. I.

PRIOR ART m F lg. 2.

PRIOR ART l3 l5 l2 /2 22 a I6 I K -24 27-- '"mm H 26 mull 2/ V X\/ /lINVENTORS F |g lsuo Uedu BY Tokoo Kogeyomo Isoo lnomotq film/1112M:

ATTORNEYS Patented July 4, 1972 3,675,165

2. Sheets-Sheet 2 INVENTORS lsoo Uedo BY Takoo Kugeyomo & lsoo lnomgtpATTORNEYS WAVEGUIDE WINDOW FOR TRANSMISSION OF ELECTROMAGNETIC WAVESThis invention relates to an input or output waveguide window for sealedmicrowave tubes and to a pressuring window for sealed high-powerwaveguide means filled with gas such as nitrogen gas.

Several types of waveguide windows for sealing waveguide systems forhigh power use are known. Among them, there are (l) half-wavelengthdielectric resonant windows; (2) cone windows; (3) slanted-disk windows;and (4) circular windows.

All these types have been widely in use as input or output windows forhigh-power microwave tubes for use in groundstations of satellitecommunications systems. Their broadband characteristics andhigh-power-withstanding property have been fully demonstrated indiversified application fields.

Nevertheless, all of these window types have caused technicaldifficulties particularly when operated at relatively low microwavefrequencies, i.e., in the UHF band.

For instance, a ceramic disk of 400 mm in diameter, is needed when awaveguide window designed for a 500 MHz band is operated as a circularwindow. This requires overall window assembly dimensions as large as 410mm in diameter and 350 mm in thickness. In case of the half-wavelengthresonant window, a ceramic block as large as 105 mm by 190.5 mm by 381mm would be required. Since the axial length of the window becomes aslong as 105 mm, the window assembly becomes bulky and heavy. Inaddition, low-loss dielectric materials used for the window such asmica, ceramic, or quartz are expensive, making both manufacture andfinishing of these dielectric materials difficult.

The dielectric-filled oval-aperture windows for low-power use areadvantageous compared with other types of windows such as the circularwindow, in that both the overall size of the window assembly and theamount of the dielectric material can be reduced. It must be notedhowever that they have a narrow bandwidth and are weak against thermalexpansion as compared with the aforementioned conventional types suitedfor high-power applications. In other words, it is substantiallyimpossible to manufacture the oval-aperture resonant windows suited forhigh-power use or large windows of this type for UHF band use.

Accordingly, it is the principal object of this invention to provide acompact, light, and inexpensive waveguide window adapted to sealingwaveguide means for handling high UHF band power, without affecting thefrequency response of the waveguide itself. I

The high-power, broad-band waveguide window of this invention is placedin a rectangular waveguide with the TE mode as the dominant mode. Thewaveguide window arrangement consists essentially of; (i) a low-lossceramic plate capable of transmitting high microwave power and of,providing excellent sealing action (i.e., nonporous, etc.) for thewaveguide system; (ii) a pair of metallic members bonded to the oppositesides of the ceramic plate. (By the provision of the metallic members onboth sides of the plate, aperture dimensions are precisely determinedand, at the same time, window failures can be prevented. If the metallicmember were bonded only to one side of the ceramic plate as in theconventional window structure, window failures due to the difference inthe thermal expansion characteristics of the two different materialswould be inevitable); (iii) an assembly of metallic members jointlyserving to clamp and support the window assembly in a rectangularwaveguide, and to provide a hermetic sealing joint for the windowaseembly; and (iv) means composed of a combination of a pair ofapertures formed on the opposite sides of the ceramic element,effectively separated with an electrical distance corresponding to theguide wavelength in the ceramic plate, and the other combination of twoor more apertures formed in the assembly of the metallic elements.

The above-mentioned and other features and advantages of this inventionwill be fully understood from the following detailed description takenin conjunction with the accompanying drawings, wherein:

FIG. 1 shows a cross section of a conventional ceramicbondedoval-aperture resonant window in a rectangular waveguide;

FIG. 2 is a plan view of the resonant window shown in FIG.

FIG. 3 shows a cross section of a resonant window for use in arectangular waveguide according this invention;

FIG. 4 shows a plan view of a ceramic plate for use in the embodiment ofFIG. 3;

FIG. 5 shows a cross section of another embodiment of this invention;and

FIG. 6 shows a characteristic curve of the embodiment of FIG. 3.

Referring to FIG. 1, it will be seen that a waveguide window of thistype is disclosed, for example, in G.L. Ragans paper published on page221 of "Microwave Transmission Circuit," Vol. 9, M.I.T. RadiationLaboratory Series.

With this window structure, the periphery of the surface of the ceramicplate 12 to form a hermetic seal with the apertured metallic plate 13 ismetallized so that the ceramic plate 12 can be brazed to the plate 13.The metallic plate 13 is securely bonded to a metallic flange 14 whichis securely clamped to the internal wall of the rectangular waveguide bybrazing.

The ceramic resonant window with a resonant opening 15 in the metallicplate 13 gives a resonance characteristic centered at a desiredoperating frequency.

Referring to the plan view of the oval-aperture resonant window of FIG.2, the aperture 15 has a shape as shown by the broken line. With thiswindow structure, a pair of confronting portions of the metallic flange14 disposed transverse to the waveguide axis and perpendicular in thedirection of the electric field acts as a parallel capacitivesusceptance element in the waveguide. In like manner, the other pair ofconfronting portions of the metallic flange 14 disposed parallel to thedirection of the electric field acts as a parallel inductive susceptancein the waveguide. Accordingly, the resonant opening 15 surrounded by thetwo pairs of confronting portions may be represented equivalently by aparallel resonant circuit.

A ceramic plate in a waveguide window is known to behave so as toincrease the above-mentioned parallel capacitance when the waveguide isshown by an equivalent circuit. Therefore, the ceramic resonant windowcan be matched to a desired resonance frequency by making the width ofthe window narrower than that of a window with an opening which is notfilled with dielectric material. Since the equivalent circuit of such aresonant window containing a dielectric element is a simple resonantcircuit without exception, such ceramic windows are narrower inbandwidth than other types of windows such as circular windows.Furthennore, with the increase in the cross sectional area of waveguidesadapted to the UI-IF band use, the size of ceramic plates becomes large.

Consequently, excessively large sealing thermal stresses are produced inthe ceramic plate due to the difference in the thermal expansioncharacteristics of the dielectric element 12 and the metallic plate 13.This tends to cause deformation or breakdown of the ceramic plate whenthe plate is in the process of fabrication or in high power operation.

Referring to the embodiment of FIG. 3, the window assembly is sealed ina rectangular waveguide 1 1. The periphery of each of the side surfacesof the ceramic plate 16 is metallized, and a pair of frame membershaving outer frame 17 and inner frame 18 are brazed to the oppositemetallized surfaces of the plate.

To the outer frame 17 one end of an inner collar member 19 is brazedsubstantially in perpendicular relations to each other. One end of anouter collar member 21 outwardly tapered is brazed to the other end ofthe inner collar 19, and the other end of the outer collar ishermetically fixed to an outer flange member 22 hermetically fixed tothe internal wall of the waveguide to form a space 28. That surface ofthe inner metallic frame 18 which is not brazed to the ceramic plate isslidably mounted on the surface of an inner flange member 23.

This window structure may be considered to contain a combination of fourresonant apertures that is, a pair of apertures 24 and 25 on oppositesides of the plate 16, an aperture 26 formed in the inner collar 19, andan aperture 27 formed in the inner flange 23.

Extensive experiments conducted by the present inventors using thisembodiment have shown the following results: (i) A maximally flatvoltage standing wave ratio (VSWR) vs. frequency characteristicobtained, which is the overall resonance characteristics of the fourapertures; (ii) despite the anticipated disadvantages such as theincrease in the overall length of the window in the axial direction ofthe waveguide, optimum VSWR ratio can be obtained by suitably selectingthe dimensions of the ceramic plate, the area of two apertures 24 and 25defined respectively by the frame members 17 and 18, and the supportingmember assembly and thereby, controlling the resonance characteristicsof the four apertures; and (iii) the apertures 24 and 25 on the oppositesides of the ceramic plate are separated with a distance equal to thethickness t of the plate. Since the dielectric coefficient of theceramic is larger than unity, however, the electrical length for thisspacing becomes longer than t (One of the apertures may be greater inarea than the other). Therefore, the resonance characteristic of thebandpass filter formed by the two apertures is governed not by thesmaller of the two, but by both. Thus, a broadband VSWR characteristiccan be obtained.

Features of this window structure may be specified as follows; (i) Thewindow is securely held and sealed in place with the aid of the innercollar 19 and the outwardly tapered outer collar 21, contiguous ends ofwhich collars are brazed or welded together. (ii) The free surface ofthe inner frame 18 is not firmly fixed to the inner flange 23 but onlypressed thereto. Thus, the deformation of the window assembly isprevented, that may be caused due to the pressure difference on bothsides of the window when the left-hand side of the ceramic plate isevacuated. (iii) Inner collar 19 and outer collar 21 are so arranged asto form space 28 which can accommodate thermal expansion of the outerframe 17 inner frame 18 and ceramic plate 16 when the ceramic plate 16is heated due to the dielectric loss. For this feature, a compact windowstructure suited for high power operation can be realized. (iv) Theouter and inner frame members 17 and 18 brazed to the oppositemetallized surfaces of the ceramic plate are of the same metallicmaterial and thickness. Consequently, a relatively uniform thermalstress distribution is obtained and the danger of deformation orbreakdown of the ceramic plate due to the difference in the thermalexpansion characteristics of the metal and the ceramic can be prevented.

It will be obvious from these features that a ceramic plate ofsufficiently large size can be brazed to the frame member and that aceramic window capable of transmitting high power can be manufactured.

A still further advantage of the window structure of FIG. 3 is that thegeometry of the plate can be arbitrarily selected. This is because theplate needs to be simply sandwiched between the frame members by brazingand is not subject to limitations of the prior art such as the geometryof the window opening or recessed part of the metallic plate.

According to the inventors experiments, the size of the ceramic plateused for a WR-ISOO type rectangular waveguide operated at 500 MHz was assmall as X I60 X 340 mm and the overall length of the window assembly inthe axial direction of the waveguide was of the order of 30 mm.

This demonstrates that the high-power, broad-band waveguide window foroperation in the UHF band can be made much more compact, lighter, andless expensive than the conventional windows.

In the embodiment shown in FIGS. 3 and 4, the outer metallic collar 21may have an outer portion extending to and hermetically fixed to theinner wall of flange member 22, replacing the inner cylindrical collar19 and the outer collar 21.

Referring now to FIG. 4, the ceramic plate 16 used in the windowassembly of FIG. 3 is of elongated-octagonal shape. The broken linerepresents the shape of the aperture 24. It is to be understood thatsuch an octagonal shape facilitates manufacture and finishing of ceramicplates and reduces the manufacturing costs as compared with theconventional ovalshaped ceramic plates, especially when the plate sizeis made large.

Referring to FIG. 5, the peripheral portion of each side surface of theceramic plate 31 is metallized similarly to the previous embodiment. Aninner frame 33 and an outer frame 320 formed integrally with anoutwardly flared collar 32 are brazed to the opposite metallizedsurfaces of the ceramic plate. The other end of the collar 32 and oneend of outer collar 34 are brazed or welded together. The other end ofthe outer collar 34 is anchored to the metallic flange 35 to form ahermetic sealing joint. The free surface of the inner frame 33.

is slidably mounted on the surface of the metallic flange 35. As in thecase of the foregoing embodiment, this window assembly contains fourresonant apertures in all that is, apertures 36 and 37 on the oppositesides of the ceramic plate, another aperture 38 in the outwardly flaredmetallic collar 32, and still another aperture 39 in the metallic flange35. It is possible with this window assembly to obtain a desiredbroadband response by suitably varying the dimensions and mutualrelationships of these apertures and thereby, controlling theirindividual rosonance characteristics.

The embodiment of FIG. 5 may be considered to be an improved version ofthe first embodiment in that the outer frame 17 and the inner collar 19in the structure of FIG. 3 have been replaced with a single-pieceoutwardly flared collar 32 and that by the adoption of this outwardlyflared collar, the flat portion of the response could be furtherextended.

Outstanding features of this embodiment are briefly summarized below.(i) Formation of space 40 surrounded by the outwardly flared innercollar 32, the outer collar 34, and the metallic flange 35 may bementioned first. This space can accommodate thermal expansion of theinner frame 33, the outer frame 32a and the ceramic plate 31 in theradial direction. (ii) The outer frame 320 and the inner frame 33 arebrazed to the opposite metallized surface of the ceramic plate 31. Thiscontributes to make uniform the thermal stress distribution in the platecaused by the different thermal expansion properties of the ceramic andmetallic materials and enables the ceramic plate to withstand the flowof high peak power. (iii) Both the combination of outer frame 17 andinner collar 19 and the combination of inner flange 23 and outer flange22 of the embodiment of FIG. 3 are united into a singlepiece unit in thewindow structure of FIG. 5. This structure facilitates mounting of theceramic plate assembly from the left-hand side waveguide section in thewindow fabrication and, at the same time, helps prevent movement of theceramic plate assembly, due to the pressure difference on both sides ofthe window when the left-hand side is evacuated.

An exact formula treating the design problem of such windows withmathematical precision does not seem to have beenpublished. Therefore,in order to obtain an optimum broadband characteristic from variousdesign data such as the thickness of the ceramic plate, the dimensionsof the apertures on both sides of the plate, the axial length of theinner collar, the dimensions of the apertures in the supporting memberassembly and the like of the window of FIG. 3, the trial and errormethod is the only approach to rely on.

Numerical data obtained with the waveguide window assembly of FIG. 3containing a high alumina ceramic plate percent pure) are as follows:

520 MHz Model WR-l500 5 X X 340 mm Center frequency: Rectangularwaveguide: Ceramic plate: Aperture of outer collar in the axialdirection: 20 mm Thus, a broad-band response with VSWR smaller than 1.20in the usable frequency range 470 570 MHz as shown in FIG. 6 has beenobtained by the combination of the four resonant apertures. A powerperformance test of this window was also conducted to evaluate the highpower withstanding property. The temperature rise of the ceramic plateat room temperature of 26 C was as mall as two or three degreescentigrade for the continuous power flow of 30 kilowatt.

Although the invention has been described with particular reference tothe two preferred embodiments, each containing a combination of fourapertures, it will be obvious to one skilled in the art that the VSWRcharacteristic vs. frequency of a desired bandwidth can be obtained bysuitably designing the window assembly so as to contain only one pair ofapertures, four apertures, or more/The two metallic frame members brazedon the opposite metallized surfaces may be of any suitable constructionother than those described in the embodiments, so far as they contributeto prevention of excessive thermal stresses as mentioned previously andreduce the possibility of consequent deformation or desu'uction of theceramic plate. Furthermore, there is provided a space in the windowassembly of any one of the embodiments for accommodating thermalexpansion of the ceramic plate assembly. There is no disadvantage,however, in incorporating any other suitable means for safely taking upthe thermal expansion. For instance, provision of a pair of apertured,corrugated diaphragms of resilient metallic material employed so as tosandwich the ceramic plate will eliminate the necessity of the provisionof such space.

it is apparent from the foregoing description that variousmodifications, and addtions of constituents of the disclosed embodimentscan be made without departing from the scope and spirit of thisinvention.

What is claimed is: 1

1. A window for sealing a waveguide filled with a preselected gas underpressure and transmitting microwave signals without adversely affectinga transmission frequency characteristic of said waveguide, comprising:

a waveguide;

member means having an outer peripheral edge hermetically fixed to aninner surface of said waveguide to project said member meansperpendicularly therefrom and having one opposite inner peripheral edgedefining a first opening disposed coaxially with a lengthwise axis ofsaid waveguide;

a dielectric plate permeable to said microwave signals and havingmetallized opposite surfaces adjacent to a periphery thereof;

a pair of frames so fixedly secured to said plate metallized peripheralopposite surfaces as to include said last-mentioned surfacestherebetween for providing on respective opposite surfaces of said platesecond and third openings having a common axis coaxial with said firstopening axis; one of said frames movably mounted on an adjacentperipheral surface of said member means proximate to said one oppositeedge portion thereof to permit movement of said plate to compensate foroccurrences of unequal gas pressures on opposite surfaces thereof; and

collar means forming a fourth opening having an axis disposed coaxiallywith said first, second and third opening axes; said collar means havingone entire peripheral edge attached to an adjacent peripheral surface ofthe other of said frames and an opposite entire peripheral edgehermetically secured to another opposite peripheral edge portion of saidmember means for providing a fifth opening between said collar means andadjacent surfaces of said one and another edge portions, portions ofsaid member means, said frames and said plate to permit movement of theother of said frames to compensate for thermal expansion of the otherframe due to microwave signal power loss in said plate;

whereby said first through fourth openings are so dimensioned as tocontrol the resonance characteristics thereof to provide said windowwith a substantially flat voltage standing wave ratio versus frequencycharacteristic over a predetermined frequency range.

2. The window according to claim 1 in which said collar means includes:

a collar forming said fourth opening and having one entire peripheraledge attached to said adjacent peripheral surface of the other frame;and

an element having one entire end fixedly positioned on an entireopposite edge of said collar and having an opposite entire peripheraledge hermetically secured to said another opposite peripheral edgeportion of said member means to provide said fifth opening.

3. The window according to claim 1 in which said collar means includes:

a collar shaping said fourth opening and having one end formedintegrally with an outer peripheral edge of the other frame and flaredoutwardly therefrom; and

an element having one entire end fixedly positioned on an opposite edgeof said collar and an entire opposite entire peripheral edgehermetically secured to said another opposite peripheral edge portion ofsaid member means to provide said fifth opening.

4. A window for sealing a waveguide filled with a preselected gas underpressure and transmitting microwave signals without adversely affectinga transmission frequency characteristic of said waveguide, comprising:

a waveguide;

member means having an outer peripheral edge hermetically fixed to aninner surface of said waveguide to project said member meansperpendicularly therefrom and having one opposite inner peripheral edgeportion defining a first opening disposed coaxially with a lengthwiseaxis of said waveguide;

a dielectric plate permeable to said microwave signals and havingmetallized opposite surfaces adjacent to a periphery thereof;

a pair of frames so fixedly secured to said plate metallized peripheralopposite surfaces as to include said last-mentioned surfacestherebetween for providing on respective opposite surfaces of said platesecond and third openings having a common axis coaxial with said firstopening axis; one of said frames movably mounted on an adjacentperipheral surface of said member means proximate to said one oppositeperipheral edge portion thereof to permit movement of said plate tocompensate for occurrences of unequal gas pressures on opposite surfacesthereof;

a collar fonning a fourth opening having an axis disposed coaxially withsaid first, second and third opening axes; said collar having an entireperipheral edge attached to an adjacent peripheral surface of the otherof said frames; and

an element having one entire end fixedly positioned on an entireopposite peripheral edge of said collar and having an entire oppositeend hermetically secured to another opposite peripheral edge portion ofsaid member means for providing a fifth opening between said collar andadjacent surfaces of said one and another edge portions of said membermeans, said frames and said plate to permit movement of the other ofsaid frames to compensate for thermal expansion of the other frame dueto microwave signal power loss in said plate;

whereby said first through fourth openings are so dimensioned as tocontrol the resonance characteristics thereof to provide said windowwith a substantially flat voltage standing wave ratio versus frequencycharacteristic over a predetermined frequency range.

5. A window for sealing a waveguide filled with a preselected gas underpressure and transmitting microwave signals without adversely affectinga transmission characteristic of said waveguide, comprising:

a waveguide;

a member having one peripheral edge hermetically fixed to an innersurface of said waveguide to project said member substantiallyperpendicularly therefrom and having an opposite peripheral edgedefining a first opening disposed coaxially with a lengthwise axis ofsaid waveguide;

a dielectric plate permeable to said microwave signals and havingmetallized opposite surfaces adjacent to a periphery thereof;

a pair of frames so fixedly secured to said plate metallized peripheralopposite surfaces as to include said last-mentioned surfacestherebetween for providing on respective opposite surfaces of said platesecond and third openings having a common axis coaxial with said firstopening axis; one of said frames movably mounted on an adjacentperipheral surface of said member proximate to said opposite edgethereof to permit movement of said plate to compensate for occurrencesof unequal gas pressures on opposite surfaces thereof;

a flared collar shaping a fourth opening having an axis disposedcoaxially with said first, second and third opening axes; said collarhaving one end formed integrally with an outer peripheral edge of theother frame; and

an element having one entire peripheral end fixedly positioned on anentire opposite end of said collar and having an entire opposite endhermetically secured to said member for providing a fifth openingbetween said collar and adjacent portions of said member, said framesand said plate to permit movement of the other of said frames tocompensate for thermal expansion of said plate due to microwave signalpower loss in said plate;

whereby said first through fourth openings are so dimentioned as tocontrol the resonance characteristics thereof to provide said windowwith a substantially flat voltage standing wave ratio versus frequencycharacteristic over a predetermined frequency range.

* i l i

1. A window for sealing a waveguide filled with a preselected gas underpressure and transmitting microwave signals without adversely affectinga transmission frequency characteristic of said waveguide, comprising: awaveguide; member means having an outer peripheral edge hermeticallyfixed to an inner surface of said waveguide to project said member meansperpendicularly therefrom and having one opposite inner peripheral edgedefining a first opening disposed coaxially with a lengthwise axis ofsaid waveguide; a dielectric plate permeable to said microwave signalsand having metallized opposite surfaces adjacent to a periphery thereof;a pair of frames so fixedly secured to said plate metallized peripheralopposite surfaces as to include said last-mentioned surfacestherebetween for providing on respective opposite surfaces of said platesecond and third openings having a common axis coaxial with said firstopening axis; one of said frames movably mounted on an adjacentperipheral surface of said member means proximate to said one oppositeedge portion thereof to permit movement of said plate to compensate foroccurrences of unequal gas pressures on opposite surfaces thereof; andcollar means forming a fourth opening having an axis disposed coaxiallywith said first, second and third opening axes; said collar means havingone entire peripheral edge attached to an adjacent peripheral surface ofthe other of said frames and an opposite entire peripheral edgehermetically secured to another opposite peripheral edge portion of saidmember means for providing a fifth opening between said collar means andadjacent surfaces of said one and another edge portions, portions ofsaid member means, said frames and said plate to permit movement of theother of said frames to compensate for thermal expansion of the otherframe due to microwave signal power loss in said plate; whereby saidfirst through fourth openings are so dimensioned as to control theresonance characteristics thereof to provide said window with asubstantially flat voltage standing wave ratio versus frequencycharacteristic over a predetermined frequency range.
 2. The windowaccording to claim 1 in which said collar means includes: a collarforming said fourth opening and having one entire peripheral edgeattached to said adjacent peripheral surface of the other frame; and anelement having one entire end fixedly positioned on an entire oppositeedge of said collar and having an opposite entire peripheral edgehermetically secured to said another opposite peripheral edge portion ofsaid member means to provide said fifth opening.
 3. The window accordingto claim 1 in which said collar means includes: a collar shaping saidfourth opening and having one end formed integrally with an outerperipheral edge of the other frame and flared outwardly therefrom; andan element having one entire end fixedly positioned on an opposite edgeof said collar and an entire opposite entire peripheral edgehermetically secured to said another opposite peripheral edge portion ofsaid member means to provide said fifth opening.
 4. A window for sealinga waveguide filled with a preselected gas under pressure andtransmitting microwave signals withouT adversely affecting atransmission frequency characteristic of said waveguide, comprising: awaveguide; member means having an outer peripheral edge hermeticallyfixed to an inner surface of said waveguide to project said member meansperpendicularly therefrom and having one opposite inner peripheral edgeportion defining a first opening disposed coaxially with a lengthwiseaxis of said waveguide; a dielectric plate permeable to said microwavesignals and having metallized opposite surfaces adjacent to a peripherythereof; a pair of frames so fixedly secured to said plate metallizedperipheral opposite surfaces as to include said last-mentioned surfacestherebetween for providing on respective opposite surfaces of said platesecond and third openings having a common axis coaxial with said firstopening axis; one of said frames movably mounted on an adjacentperipheral surface of said member means proximate to said one oppositeperipheral edge portion thereof to permit movement of said plate tocompensate for occurrences of unequal gas pressures on opposite surfacesthereof; a collar forming a fourth opening having an axis disposedcoaxially with said first, second and third opening axes; said collarhaving an entire peripheral edge attached to an adjacent peripheralsurface of the other of said frames; and an element having one entireend fixedly positioned on an entire opposite peripheral edge of saidcollar and having an entire opposite end hermetically secured to anotheropposite peripheral edge portion of said member means for providing afifth opening between said collar and adjacent surfaces of said one andanother edge portions of said member means, said frames and said plateto permit movement of the other of said frames to compensate for thermalexpansion of the other frame due to microwave signal power loss in saidplate; whereby said first through fourth openings are so dimensioned asto control the resonance characteristics thereof to provide said windowwith a substantially flat voltage standing wave ratio versus frequencycharacteristic over a predetermined frequency range.
 5. A window forsealing a waveguide filled with a preselected gas under pressure andtransmitting microwave signals without adversely affecting atransmission characteristic of said waveguide, comprising: a waveguide;a member having one peripheral edge hermetically fixed to an innersurface of said waveguide to project said member substantiallyperpendicularly therefrom and having an opposite peripheral edgedefining a first opening disposed coaxially with a lengthwise axis ofsaid waveguide; a dielectric plate permeable to said microwave signalsand having metallized opposite surfaces adjacent to a periphery thereof;a pair of frames so fixedly secured to said plate metallized peripheralopposite surfaces as to include said last-mentioned surfacestherebetween for providing on respective opposite surfaces of said platesecond and third openings having a common axis coaxial with said firstopening axis; one of said frames movably mounted on an adjacentperipheral surface of said member proximate to said opposite edgethereof to permit movement of said plate to compensate for occurrencesof unequal gas pressures on opposite surfaces thereof; a flared collarshaping a fourth opening having an axis disposed coaxially with saidfirst, second and third opening axes; said collar having one end formedintegrally with an outer peripheral edge of the other frame; and anelement having one entire peripheral end fixedly positioned on an entireopposite end of said collar and having an entire opposite endhermetically secured to said member for providing a fifth openingbetween said collar and adjacent portions of said member, said framesand said plate to permit movement of the other of said frames tocompensate for thermal expansion of said plate due to microwave signalpower loss in said plate; whereby said first through fourTh openings areso dimentioned as to control the resonance characteristics thereof toprovide said window with a substantially flat voltage standing waveratio versus frequency characteristic over a predetermined frequencyrange.